The removal and treatment of pollutants in industrial gas streams has resulted in the development of many types of pollution control devices. One commonly used device is a rotary concentrator. A rotary concentrator is basically a rotating unit enclosed within a housing. The rotating unit includes adsorption media, such as activated carbon or zeolite depending on the pollutant to be removed from the inlet gas stream. There are various types of rotary concentrators, including concentrators wherein the adsorbent media is disk-shaped or in the form of an annulus and concentrators which utilize blocks of media arranged, for example, around a central axis to form an annulus. Generally, the media has a porous ceramic base or substrate with the appropriate adsorbent media deposited on the base. However, the rotating unit and media can take various forms including layered disks, etc. as would be known to a person of ordinary skill in this art.
In a conventional rotary concentrator, the adsorbent media is divided by ducting and seals into an adsorbent section or zone and a smaller desorbent zone. Inlet process gas, which contains an adsorbable pollutant, is passed over the adsorbent media in the adsorption zone, which removes the adsorbable pollutants. Clean gas then leaves the rotary concentrator. Heated desorption gas is simultaneously passed over the adsorbent media in the desorbent zone, thereby removing the pollutants, which can be destroyed or oxidized in a thermal or catalytic oxidizer for example. Thus, as the media rotates within the housing, the process air is cleaned of adsorbable pollutants and the adsorbable pollutants are removed from the media in the desorption zone.
The temperature of the gas necessary for desorption will depend upon the media and the adsorbable pollutants. In a typical application, the desorption gas is heated to a temperature of between about 250.degree. to 350.degree. F. which effectively removes the adsorbed pollutants in the desorption zone. Generally, clean gas is heated to the desorption temperature of the media and the gas is then circulated to an oxidizer processor. The construction and operation of conventional rotary concentrators are well known in this art and therefore no detailed description of such concentrators is required for the understanding of the improved rotary concentrator and method of this invention.
Fig. 1 illustrates schematically a conventional rotary concentrator. As set forth above, a conventional rotary concentrator includes adsorbent media 20 which is rotated within a sealed housing (not shown). The housing includes ducting and seals which direct the process inlet gas stream to predetermined sections of the adsorbent media as now described. As set forth above, the adsorbent media 20 can take various forms, including a disk-shaped, an annular shaped, blocks, etc. However, the shape and configuration of the housing and the form of the adsorbent media is well known in this art and does not form a part of this invention. The adsorbent media is divided by the housing and seals into an adsorbent zone 22 and a much smaller desorbent zone 24. Contaminated gas containing an adsorbable pollutant is directed over the adsorbent media in the adsorbent zone 22 as shown in FIG. 1 at 26. Where the adsorbent media 20 is disk-shaped as shown, annual or blocks of media, the media is normally applied to a substrate having numerous holes and interticies. The gas then flows through the interticies in the substrate over the adsorbent media. The adsorbable pollutants are then adsorbed by the media, such that the gas leaving the adsorbent zone 22 has been purified by removal of the adsorbable pollutants as shown at 28. The adsorbed pollutants must then be removed from the adsorbent media, which is accomplished in the desorbent zone 24.
As the adsorbent media 20 rotates as shown by arrow 30, a typical segment 32 will rotate into the desorbent zone 24. As will be understood, this segment now contains adsorbable pollutants which must be removed from the adsorbent media. The absorbed pollutants are then removed in the desorbent zone 24 by hot desorbent gas, such as air heated to the desorption temperature. The desorption temperature will depend upon the pollutant which has been absorbed by the media and the media used. In a typical application, the desorption temperature is between about 250.degree. to 350.degree. F. The hot desorption gas 34 enters the desorption zone 24 and is passed over the adsorbent media, removing the pollutants from the media. The desorption gas is then directed to a control device (not shown) through a desorption gas outlet 36 which removes or destroys the pollutant, typically a thermal oxidizer, such as a regenerative thermal oxidizer or a catalytic oxidizer. The oxidizer processor typically vents the hot clean air to the atmosphere. Rotary concentrators also may include an optional cooling zone 38 shown in broken lines to cool the adsorbent media prior to returning the media for use for absorption. That is, the cooling zone 38 is not a desorbent zone because the pollutants have already been removed from the desorbent zone 24 prior to movement of the adsorbent media into the cooling zone 38. As will be understood, the media is more adsorbent at cooler temperatures. The clean coolant gas is directed from the inlet 40, over the media in the cooling zone 38, to the outlet 42.
FIG. 2 is a graph of the temperature of air exiting a segment of media as it passes through the desorbent zone 24 and the pollutant concentration as the media rotates through the desorbent zone. The adsorbent media is initially at the adsorbent temperature, which is generally between ambient and about 100.degree. F. The media is then heated to the desorption temperature by the hot desorption gas as shown at 34 in FIG. 1. Typically, the desorption temperature is between 250.degree. to 350.degree. F. in a conventional rotary concentrator. Thus, the media is heated to the desorption temperature and as it approaches the desorption temperature, the concentration of the adsorbable pollutants peak and fall off in a bell-shaped curve. As will be understood from FIG. 4, the media 20 in the desorption zone 24, however, remains essentially at the desorption temperature until cooled either in the cooling zone 38 or by the inlet polluted gas stream. Thus, FIG. 2 illustrates the temperature and concentration of pollutants exiting from a typical segment 32 as it moves through the desorption zone 24. It has been believed, however, that it is necessary to maintain the desorption temperature of the media in the desorption zone 24 until all of the pollutants are removed by the heated desorption gas.
Reference is also made to U.S. Pat. No. 5,788,744 assigned to the assignee of the present application. This patent discloses an improved rotary concentrator, wherein the gas received from the desorption zone is divided into two outlet sections. The first section which is located upstream of the desorption zone is relatively free of pollutant gas and recirculated either to the desorption or adsorption inlet. The balance of the desorption exhaust gas has a lower volume, higher pollution concentration and a higher temperature, all of which lead to more cost effective final control. However, the split desorption outlet system disclosed in this patent requires the same volume of heated desorption gas as a conventional rotary concentrator system. The energy and equipment required to provide this volume of heated gas limited the capital and operating cost advantages of the split desorption outlet system as compared to a conventional rotary concentrator system.
The improved rotary concentrator system and method of this invention utilizes a dual temperature and preferably a dual desorption zone that significantly reduces the volume of heated adsorption gas required. The rotary concentrator and method of this invention is thus more efficient and cost effective.